Behn
Mark D.
Behn
Mark D.
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ArticleMantle flow and melting underneath oblique and ultraslow mid-ocean ridges(American Geophysical Union, 2007-12-25) Montesi, Laurent G. J. ; Behn, Mark D.Mid-ocean ridge morphology correlates strongly with spreading rate. As the spreading rate decreases, conductive cooling becomes more important in controlling ridge thermal structure and the axial lithosphere thickens. At ultraslow spreading rates, the ridge axis becomes sufficiently cold that peridotite blocks are emplaced directly at the seafloor and volcanism is limited to localized volcanic centers widely spaced along the ridge axis. Some slow-spreading ridges adopt an ultraslow morphology when their axis is oblique to the spreading direction. We present an analytical solution for mantle flow beneath an oblique ridge and demonstrate that the thermal structure and crustal thickness are controlled by the effective spreading rate (product of the plate separation velocity and the cosine of obliquity). A global compilation of oblique ridges reveals that ultraslow morphology corresponds to effective half rates less than 6.5 mm/yr, resulting in lithosphere that is thicker than ~30 km. We conclude that the transition from slow to ultraslow spreading is not related to a change of melt productivity but rather in the efficiency of vertical melt extraction.
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ArticleGrain-size dynamics beneath mid-ocean ridges : implications for permeability and melt extraction(John Wiley & Sons, 2015-03-26) Turner, Andrew J. ; Katz, Richard F. ; Behn, Mark D.Grain size is an important control on mantle viscosity and permeability, but is difficult or impossible to measure in situ. We construct a two-dimensional, single phase model for the steady state mean grain size beneath a mid-ocean ridge. The mantle rheology is modeled as a composite of diffusion creep, dislocation creep, dislocation accommodated grain boundary sliding, and a plastic stress limiter. The mean grain size is calculated by the paleowattmeter relationship of Austin and Evans (2007). We investigate the sensitivity of our model to global variations in grain growth exponent, potential temperature, spreading-rate, and mantle hydration. We interpret the mean grain-size field in terms of its permeability to melt transport. The permeability structure due to mean grain size may be approximated as a high permeability region beneath a low permeability region. The transition between high and low permeability regions occurs across a boundary that is steeply inclined toward the ridge axis. We hypothesize that such a permeability structure generated from the variability of the mean grain size may focus melt toward the ridge axis, analogous to Sparks and Parmentier (1991)-type focusing. This focusing may, in turn, constrain the region where significant melt fractions are observed by seismic or magnetotelluric surveys. This interpretation of melt focusing via the grain-size permeability structure is consistent with MT observation of the asthenosphere beneath the East Pacific Rise.
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PreprintTopographic controls on dike injection in volcanic rift zones( 2006-04-03) Behn, Mark D. ; Buck, W. Roger ; Sacks, I. SelwynDike emplacement in volcanic rift zones is often associated with the injection of “bladelike” dikes, which propagate long distances parallel to the rift, but frequently remain trapped at depth and erupt only near the tip of the dike. Over geologic time, this style of dike injection implies that a greater percentage of extension is accommodated by magma accretion at depth than near the surface. In this study, we investigate the evolution of faulting, topography, and stress state in volcanic rift zones using a kinematic model for dike injection in an extending 2-D elastic-viscoplastic layer. We show that the intrusion of blade-like dikes focuses deformation at the rift axis, leading to the formation of an axial rift valley. However, flexure associated with the development of the rift topography generates compression at the base of the plate. If the magnitude of these deviatoric compressive stresses exceeds the deviatoric tensile stress associated with far-field extension, further dike injection will be inhibited. In general, this transition from tensile to compressive deviatoric stresses occurs when the rate of accretion in the lower crust is greater than 50-60% of the far-field extension rate. These results indicate that over geologic time-scales the injection of blade-like dikes is a self-limiting process in which dike-generated faulting and topography result in an efficient feedback mechanism that controls the time-averaged distribution of magma accretion within the crust.
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ArticleMagmatic and tectonic extension at mid-ocean ridges : 2. Origin of axial morphology(American Geophysical Union, 2008-09-30) Ito, Garrett T. ; Behn, Mark D.We investigate the origin of mid-ocean ridge morphology with numerical models that successfully predict axial topographic highs, axial valleys, and the transition between the two. The models are time-dependent, simulating alternating tectonic and magmatic periods where far-field extension is accommodated by faulting and by magmatism, respectively. During tectonic phases, models predict faults to grow on either side of the ridge axis and axial height to decrease. During magmatic phases, models simulate magmatic extension by allowing the axial lithosphere to open freely in response to extension. Results show that fault size and spacing decreases with increasing time fraction spent in the magmatic phase F M . Magmatic phases also simulate the growth of topography in response to local buoyancy forces. The fundamental variable that controls the transition between axial highs and valleys is the “rise-sink ratio,” (F M /F T )(τ T /τ M ), where F M /F T is the ratio of the time spent in the magmatic and tectonic periods and τ T /τ M is the ratio of the characteristic rates for growing topography during magmatic phases (1/τ M ) and for reducing topography during tectonic phases (1/τ T ). Models predict the tallest axial highs when (F M /F T )(τ T /τ M ) ≫ 1, faulted topography without a high or valley when (F M /F T )(τ T /τ M ) ∼ 1, and the deepest median valleys when (F M /F T )(τ M /τ T ) < 1. New scaling laws explain a global negative correlation between axial topography and lithosphere thickness as measured by the depths of axial magma lenses and microearthquakes. Exceptions to this trend reveal the importance of other behaviors such as a predicted inverse relation between axial topography and spreading rate as evident along the Lau Spreading Center. Still other factors related to the frequency and spatial pervasiveness of magmatic intrusions and eruptions, as evident at the Mid-Atlantic and Juan de Fuca ridges, influence the rise-sink-ratio (F M /F T )(τ T /τ M ) and thus axial morphology.
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ArticleRole of melt supply in oceanic detachment faulting and formation of megamullions(Geological Society of America, 2008-06) Tucholke, Brian E. ; Behn, Mark D. ; Buck, W. Roger ; Lin, JianNormal faults are ubiquitous on mid-ocean ridges and are expected to develop increasing offset with reduced spreading rate as the proportion of tectonic extension increases. Numerous long-lived detachment faults that form megamullions with large-scale corrugations have been identified on magma-poor mid-ocean ridges, but recent studies suggest, counterintuitively, that they may be associated with elevated magmatism. We present numerical models and geological data to show that these detachments occur when ~30%–50% of total extension is accommodated by magmatic accretion and that there is significant magmatic accretion in the fault footwalls. Under these low-melt conditions, magmatism may focus unevenly along the spreading axis to create an irregular brittle-plastic transition where detachments root, thus explaining the origin of the enigmatic corrugations. Morphological and compositional characteristics of the oceanic lithosphere suggested by this study provide important new constraints to assess the distribution of magmatic versus tectonic extension along mid-ocean ridges.
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ArticleMagmatic focusing to mid-ocean ridges : the role of grain-size variability and non-Newtonian viscosity(John Wiley & Sons, 2017-12-06) Turner, Andrew J. ; Katz, Richard F. ; Behn, Mark D. ; Keller, TobiasMelting beneath mid-ocean ridges occurs over a region that is much broader than the zone of magmatic emplacement that forms the oceanic crust. Magma is focused into this zone by lateral transport. This focusing has typically been explained by dynamic pressure gradients associated with corner flow, or by a sublithospheric channel sloping upward toward the ridge axis. Here we discuss a novel mechanism for magmatic focusing: lateral transport driven by gradients in compaction pressure within the asthenosphere. These gradients arise from the covariation of melting rate and compaction viscosity. The compaction viscosity, in previous models, was given as a function of melt fraction and temperature. In contrast, we show that the viscosity variations relevant to melt focusing arise from grain-size variability and non-Newtonian creep. The asthenospheric distribution of melt fraction predicted by our models provides an improved explanation of the electrical resistivity structure beneath one location on the East Pacific Rise. More generally, we find that although grain-size and non-Newtonian viscosity are properties of the solid phase, their effect on melt transport beneath mid-ocean ridges is more profound than their effect on the mantle corner flow.
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ArticleMelt generation, crystallization, and extraction beneath segmented oceanic transform faults(American Geophysical Union, 2009-11-13) Gregg, Patricia M. ; Behn, Mark D. ; Lin, Jian ; Grove, Timothy L.We examine mantle melting, fractional crystallization, and melt extraction beneath fast slipping, segmented oceanic transform fault systems. Three-dimensional mantle flow and thermal structures are calculated using a temperature-dependent rheology that incorporates a viscoplastic approximation for brittle deformation in the lithosphere. Thermal solutions are combined with the near-fractional, polybaric melting model of Kinzler and Grove (1992a, 1992b, 1993) to determine extents of melting, the shape of the melting regime, and major element melt composition. We investigate the mantle source region of intratransform spreading centers (ITSCs) using the melt migration approach of Sparks and Parmentier (1991) for two end-member pooling models: (1) a wide pooling region that incorporates all of the melt focused to the ITSC and (2) a narrow pooling region that assumes melt will not migrate across a transform fault or fracture zone. Assuming wide melt pooling, our model predictions can explain both the systematic crustal thickness excesses observed at intermediate and fast slipping transform faults as well as the deeper and lower extents of melting observed in the vicinity of several transform systems. Applying these techniques to the Siqueiros transform on the East Pacific Rise we find that both the viscoplastic rheology and wide melt pooling are required to explain the observed variations in gravity inferred crustal thickness. Finally, we show that mantle potential temperature Tp = 1350°C and fractional crystallization at depths of 9–15.5 km fit the majority of the major element geochemical data from the Siqueiros transform fault system.